The present invention pertains generally to ultra violet (uv) spectrum irradiation of overvarnish layers applied to containers and more particularly to radiation curing of cans to provide uniform curing of both inner and outer portions of the cans without rotation.
Beverage containers are normally formed from either two-piece containers or three-piece containers. Two-piece containers comprise a lid portion and a can body portion. In many cases, can body portions are formed by a cupping and ironing process utilizing sheet metal, such as sheet aluminum, which is lubricated with an oil/water emulsion coolant and then stamped into relatively short height and relatively large diameter cups. In the body making step, each cup is forced by a moving ram through a series of concentric ironing dies that stretch the cup to form a relatively taller height and relatively smaller diameter can body, approximating the size of the finished product cans. The can bodies are then trimmed and carried through a multiple stage can body washer which removes used coolant and metal fines from the can body. This process is more fully described in U.S. Pat. No. 4,027,685 issued June 7, 1977 to Heard et al, which is specifically incorporated herein by reference for all that it discloses.
After the can bodies are cleaned and dried in the multiple stage can body washer, the can bodies are sent to a decorator machine which applies decorative layers to the outer cylindrical surface of the can body, as generally described in U.S. Pat. No. 4,267,771 issued to James S. Stirbis on May 19, 1981, which is specifically incorporated herein by reference for all that it discloses. After the decorative layers are applied to the can body, an ultraviolet photoreactive overvarnish layer is applied to the entire outer cylindrical surface of the can body over the decorative layers. This overvarnish layer provides a protective layer to the decorative layers and also produces a shiny and aesthetic appearance to the decorative layers and aluminum surfaces of the outer cylindrical portions of the can body which have not been coated with a decorative layer. From the decorator machine the can bodies are then loaded onto a pin chain which supports the can bodies from the inner cylindrical surface. The can bodies then pass through an ultraviolet oven which irradiates the outer cylindrical surfaces of the can with ultraviolet radiation such as disclosed in U.S. Pat. No. 3,147,363 issued to Mohn et al. on Sept. 1, 1964, U.S. Pat. No. 3,733,709 issued to Bassemir on May 22, 1973, U.S. Pat. No. 3,745,307 issued to Peek et al. on July 10, 1973, U.S. Pat. No. 3,826,014 issued to Helding on July 30, 1974, U.S. Pat. No. 3,914,594 issued to Helding on Oct. 21, 1975 and U.S. Pat. No. 3,983,039 issued to Eastland on Sept. 28, 1976, which are specifically incorporated herein by reference for all that they disclose.
The photoreactive overvarnish coating applied by the decorator machine is specially formulated for curing by uv radiation in a process of uv photopolymerization. A particular advantage of using uv photoreactive coatings is that uv radiant sources can be easily employed in a high speed assembly line process. During application of the uv photoreactive overvarnish layer to the outer cylindrical portions of the can body, inner cylindrical portions as well as inner bottom portions of the can body also become coated, to some extent, by the overvarnish material. Since the uv photoreactive overvarnish material does not cure by simply allowing it to dry in the air, but only in response to uv radiation, the inner cylindrical portions and inner bottom portions of the can body must be irradiated with uv radiation to insure curing. Otherwise, the can body will be transported to the next stage of the can manufacturing process where a protective coating (FDA coating) is applied to the inner portions of the can to isolate the aluminum surfaces of the can body from the beverage to be contained within the can body. These protective coatings cannot be applied and uniformly cured on the inner surfaces of the can body if the overvarnish layer has not been previously cured. Consequently, uv radiation must be applied uniformly to both outside and inside portions of the can body to uniformly cure the overvarnish layer before the can bodies proceed to the next step in the fabrication process, since the photoreactive overvarnish coatings will not otherwise cure prior to application of additional layers such as the protective FDA inner coating.
In order to provide uniform curing of the outside portions of a cylindrical object, prior art devices have utilized pins which rotate the can as the can passes the radiation curing oven, such as disclosed in U.S. Pat. No. 3,840,999 issued to Whelan on Oct. 15, 1974, U.S. Pat. No. 3,894,237 issued to Choate et al. on July 8, 1975, and U.S. Pat. No. 4,129,206 issued to Talbott on Dec. 12, 1978, which are specifically incorporated herein by reference for all that they disclose. The can is then removed from the rotating pin and placed in a track so that the inside of the container can be irradiated by a separate oven. This arrangement requires numerous handling steps, including the use of several elevators and a complex rotating pin arrangement to achieve uniform curing of the uv photoreactive overvarnish layers.
To overcome these disadvantages and limitations, systems have been developed, such as disclosed in U.S. Pat. No. 4,208,587 by Eastland et al which provides a device for curing cylindrical objects, such as beverage containers, without rotation. Eastland et al utilizes a radiation source, such as a uv lamp, in a curved reflector which focuses radiant energy from the uv lamp onto a focal plane. The can body is then transported past the radiation source between the focal plane and the radiant source so that radiation impinges upon the cylindrical object before the radiation reaches the focal plane. In this manner, Eastland et al attempts to provide a broader distribution of radiation on the surface of the can body in an effort to achieve a more uniform curing of the outer cylindrical surfaces of the can body without rotation.
The disadvantages of non-rotating systems, such as disclosed by Eastland et al, are that in-line portions of the outer cylindrical surface of the can body, i.e., the outer portions of the can body that do not directly face the source of radiation, frequently do not receive sufficient radiant energy to achieve uniform curing of the uv photoreactive overvarnish layer. Non-uniform curing results from the inability of the radiation source and the reflector associated therewith to direct sufficient radiant energy onto the in-line portions of the can body. Consequently, the use of such non-rotating curing devices frequently results in the production of undercured coating on the outer cylindrical surface of the can body.
Moreover, such systems do not eliminate the necessity for use of additional ovens, the hardware associated therewith and the numerous handling steps required to cure the inner portions of the can body. In other words, although use of rotating pins and associated hardware is eliminated in non-rotating systems such as disclosed by Eastland et al, such systems frequently result in undercured outer surfaces of the can body and do not eliminate the necessity for an additional oven for curing the inner portions of the can body and the additional hardware and handling steps required.
Consequently, it would be advantageous to provide a system which is capable of uniformly curing uv photoreactive overvarnish layers on the outer cylindrical portions of can bodies, as well as inner portions of the can bodies in a simple one-step process which eliminates the complex hardware and handling steps required by conventional prior art devices.